1. Field of the Invention
The invention generally relates to a mask for photolithographic manufacturing and, more particularly to method for etching the mask by using a gas mixture having a high selectivity between an opaque layer and a resist layer.
2. Background Description
Patterning a thin film layer to a desired pattern is important in various industries, for example, semiconductor integrated circuit fabrication. To achieve this goal, the target layer is overlapped with a mask having a corresponding pattern and exposed to an illumination source, for example, light, x-ray, e-beam, etc. In photolithography for semiconductor fabrication, it has been a common practice to use a chromium (Cr) mask as a light shield which is a chromium film in a specific pattern formed on a quartz substrate. The patterning of a chromium film is usually started with a photolithographic process, during which a quartz substrate with a chromium film is prepared, a resist film is coated on the chromium film, the resist film is patterned with an electron beam to form a resist pattern. Upon completing the photolithographic process, an etching process is followed to pattern the chromium film.
In recent pursuit of micro-fabrication, there has been increasing demand for proximity correction masks, in which the dimensions of fabrication are smaller than the wavelength of exposing light. In other words, in photolithography for patterning with dimensions smaller than the wavelength of exposing light, it is necessary to accurately control the intensity of light or the amount of light passing through the opening and the diffraction of light. This control is accomplished by a proximity effect correcting mask with accurate fine patterns which do not form images on the wafer. Thus, to achieve higher resolution, finer patterning of the photomask is necessary.
Dry etching is commonly used since it significantly improves patterned shapes (e.g., edge roughness and profile) and resolution of fine patterns as compared to wet etching. Presently, a gas mixture of oxygen and chlorine is widely used as an etchant for dry etching chromium masks. In certain applications, helium is added to the mixture for plasma stabilization and uniformity. However, the gas mixture of chlorine and oxygen exhibits a low selectivity (e.g., 1:1) between the chromium thin film and a resist. The selectivity is further sacrificed when helium (He) is added for plasma stabilization and uniformity. Also, the oxygen element in the gas mixture tends to etch the resist at an undesirably high rate. Thus, the resist must be formed thick enough to avoid any overetching problems. For example, in order to pattern a chromium layer having a thickness of about 1000 Å, a resist should be formed at the thickness of at least 3000 Å to be safe from the overetching problems. This undesirably thick resist degrades the lution and accuracy of the chromium pattern during the dry etching.
Also, it has been observed that the selectivity between chromium and a resist is highly sensitive to the amount of chromium exposed to the etchant gas mixture of chlorine and oxygen during the etching process. For example, the selectivity of the gas mixture of chlorine and oxygen sharply declines as a larger area of the chromium layer is exposed to the etchant, which is commonly known as a macro-loading effect. Thus, in order to accurately pattern a larger area of chromium, a significantly larger amount of the etchant is required. However, the resist is less susceptible to the macro-loading effect than chromium and therefore an unnecessarily large amount of the resist can be stripped off due to the excessive amount of the etchant gas mixture, thereby causing overetching problems.
Further, it has been known that chromium exposed adjoining a highly populated resist pattern is etched less than chromium exposed far from the highly populated resist pattern, which is known as a pattern density effect. It has been observed that the gas mixture of chlorine and oxygen is susceptible to the pattern density effect, thereby deterring etching uniformity. For example, during the resist deposition step, a wide resist frame is sometimes formed along the edges of the substrate, and chromium exposed adjoining the wide resist frame is significantly underetched due to the pattern density effect. Therefore, there is a need for effective methodology for patterning a photomask with higher resolution and accuracy with etching uniformity.